Amplifier circuit for ultra short waves



Oct. 26, 1943. w. KLEEN 2,332,919

AMPLIFIER CIRCUIT FOR ULTRA-SHORT WAVES Filed June 5, 1941 INVENTOR WERNER kl. EEN

ATTORNEY Patented Oct. 26, 1943 AMPLIFIER CIRCUIT FOR ULTRA SHORT WAVES Werner Kleen, Berlin, Germany; vested in the Alien Property Custodian Application June 3, 1941, Serial No. 396,379

In Germany November 23, 1939 2 Claims.

In all amplifier circuits the reaction by the plate alternating potential by virtue of the gridplate capacitance upon the control grid is extremely annoying, for it means that the input and the output circuits are no longer independent of each other; indeed, they are coupled by an impedance the size of-.which is determined by the grid-plate capacitance. The plate reaction is particularly strong in the case of ultra-short waves, that is, waves of a length less than 5 meters, for the reason that in this wave band the alternating-current resistance of the grid-plate capacitance is extremely low and thus the coupling between input and output circuits very strong. Now, the invention hereinafter to be described is suited to completely eliminate the plate reaction.

The plate reaction must be obviated entirely if the occurrence of an alternating potential at the plate is wholly avoided, and this is feasible if the plate circuit is designed in the form of a series resonant circuit tuned to the operating frequency and which consists of an inductance and a capacity. The amplified potential may be derived from across one of the two elements of the series resonant circuit and fed to the input circuit of the next stage. Preferably the series resonant circuit is associated with the tube in such a way that the inductance is united with the plate and the capacity with the cathode, the amplified potential being derived from across the capacity. This arrangement is expedient for the reason that the input capacitance of the following tube is connected in parallel to the capacity of the series-resonant circuit or may act as such.

The invention will now be described in connection with the accompanying drawing in which Figs. 1, 2 and 3 disclose circuits embodying various modifications according to the invention.

Referring to Fig. 1, there is shown the tube I upon the control grid of which, that is, across the terminals a and b, there is impressed the alternating potential Ug to be amplified, while by way of a high resistance grid leak Rg a grid biasing voltage is impressed. Between plate and cathode is the series resonant circuit comprising the inductance L1 and the capacity C and tuned to the input alternating potential Ug. The D. C. plate voltage is supplied through a choke coil D. In-

fit)

asmuch as the series resonant circuit constitutes a short-circuit for the operating frequency, no alternating potential at all will arise at the plate of tube I, and this means that reaction by the plate alternating voltage through the grid-plate capacitance Cga upon the control grid is entirely precluded. The amplified alternating potential is taken off at the capacity C and then through a coupling condenser Ck which represents a negligibly low resistance for the operating frequency, is impressed upon the control grid of the following tube 2. Also the control grid of the latter tube, through a high-ohm leak resistance Rg is impressed with a negative biasing voltage for the purpose of fixing the operating point.

In the amplification of ultra-short waves, that is, wave-lengths less than 5 meters, the foregoing circuit organization fails to furnish sufficient amplification, as can be demonstrated by the following considerations: Capacitor C is connected in parallel relation to the input capacity of tube 2. The capacity acting in the series resonant circuit is at least equal to the input capacity of tube 2, and this capacity as a general rule can not be made less than 3 mi. It is known in the art that the alternating-current resistance Z of a capacity, for a wave-length l, is calculable by this formula:

where A is in terms of meters, and C in mmf. while Z is in ohm values. For A=1 meter and C=3 mmf. there follows that Z=180 ohms. For

the alternating potential Uc which arises at the capacity C there holds good the relation Uc=Sm. Z. Ug. Inasmuch as the slope Sm (mutual conductance), in the conventional kind of shortwave tube, may be taken to be around 2 mA/V, there results for the case cited Uc=.36U In other words, tube I fails to give gain; in fact, the input alternating potential Ugis reduced. However, this drawback may be obviated by the step as hereinafter outlined.

In an amplifier circuit organization for ultrashort waves in which the output circuit is a series resonant circuit and in which the output potential for the following stage is taken from across the capacity of the series resonant circuit, the capacitive element or component of the series resonant circuit according to the invention, consists of the input capacity of the next tube and a capacity optionally connected in parallel relation thereto on the one hand, and an inductance connected in parallel relation thereto on the other hand, the inductance being chosen of such dimension that together with the capacity in paralle1 thereto it represents a capacitive 'impedance (reactance) which is essentially smaller than the input capacity. This shall now be exand optionally an additional capacity C is so detuned in reference to the frequency to be amplified, in other words, is tuned to a so much lower frequency, that the fly-wheel circuit, for the operating frequency, acts like a capacity which is far lower than the input capacity Ce. Proper tuning may be effected, for instance, by variation of the capacity C, unless it is preferred to dispense with the capacity C and to use for the capacitive component of the serieslresonant circuit only the input capacity Ce. In this latter instance, the tuning of the fly-wheel circuit CeLZ is brought about either by suitable choice of the inductance L2 or else by variation of the input capacity Ce. The latter scheme is feasible for the reason that the input capacity is a function of the discharge density between the cathode and the control grid so that it is open to action in a circuitous way through the discharge current of tube 2. Hence, if the fiy-wheel circuit operative between the control grid and the oathode of tube 2 is tuned in such a way that it represents for the operating frequency a capacity of,

say, 0.3 mmf., in the light of the data and relations before-mentioned, for a wave-length of 1 meter, this corresponds to an impedance of 1800 ohms. Assuming a slope of 2 mA/V, an input potential results at the grid of tube 2 which is 3.6 times higher than the alternating potential Ug fed to the grid of tube I; in other words, the voltage gain is 3.6.

Fig. 3 shows another embodiment of the circuit organization of this invention in which the input capacity Ce serves as the capacity of the series resonant circuit, of which by means of the inductance L2 part is tuned away. In the form of construction here shown the blocking condenser Ck is inserted in the lead brought to the control grid of the tube 2 in such a way that the additional inductance L2 is included in the D. C. plate circuit of tube I. However, the blocking condenser could also be connected between L1 and L2 so that the additional inductance L2, in

respect to D. C. pertains to the grid circuit of tube 2.

In designing the circuit organization of the invention it is recommendable to use tubes fitted with a screen grid between control grid and plate, or, optionally, fitted with an additional suppressor grid between the screen grid and the plate. If the series resonance circuit hereinbefore described which is used to act as an external resistance had no damping, no alternating potential would arise between plate and cathode of the tube; hence, no reaction would arise regardless of what the value of the grid-plate capacitance might be. However, inasmuch as damping in the series resonant-circuit is inevitable, especially where short waves are dealt with, a small alternating potential will always be present. For this reason it will be expendient, in order to minimize the reaction by way of 03a, to employ a pentode instead of a triode, for in this case the value of Cga would be several orders ofmagnitude lower than for a triode.

What I claim is:

1. Amplifier circuit for ultra-short waves, in which the output circuit of the first stage is in the form of a series resonant circuit tuned to the operating frequency, and in which the input potential for the following stage is derived from across the capacity of the series resonant circuit, with the characteristic feature that the capacitive component of the series resonant circuit is formed by the input capacity of the following tube and a capacitance connected in parallel relationship thereto, and further an in ductance arranged in parallel to it, the inductance being proportioned in such a way that together with the capacity paralleled thereto it represents a capacitive impedance which is substantially lower than the input capacity of the following tube.

2. Amplifier circuit for ultra-short waves comprising first and second tubes, a series-resonant circuit consisting of inductance and capacity forming the output circuit of the first tube, the capacity of said series-resonant circuit being constituted by the grid to cathode capacity of the second tube, a second inductance connected in shunt to the capacity of the series-resonant circuit, and a coupling condenser connected from the common terminal of the resonant circuit elements to the grid of the second tube.

WERNER KLEEN. 

